This application relates to a method of operating a brushless DC motor, wherein an open loop control is used at low speed, and utilizes sinusoidal commutation, and a closed loop control is utilized at higher speeds and utilizes six step commutation.
Motor controls are becoming quite complex. In particular, sensorless controls for electric motors must commutate the motor windings using a three-phase variable voltage variable frequency power supply. Complex feedback of speed and position as well as current is provided to ensure that the motor is operating as desired.
Typically, at lower speeds, an open loop controller is utilized. At higher speeds, the control switches to closed loop control. It is typical in the prior art that each of these controls communicate through a mode selector switch, and that each provides six step or 120° commutation.
However, at lower speeds, it is difficult to sense the back emf phenomenon, which is utilized to provide much of the feedback. For this reason, the open loop control is used at low speed, however, the open loop control is sensitive to the torque fluctuations which may sometimes cause the control to misidentify the actual position or speed of the motor.